While grinding item world is a good use of your time, this is not really the best way to do it.
You'll almost always get one or two pieces of equipment that are better than the one you're working on, by the time you get it to max level.
According to Japanese players, the drop rate is 25 percent for a general, 45 percent for a kind and like 87.5 percent for a god. So, if you auto 30 levels you might get one piece, but it's not uncommon to get zero.
The best fist I have available in the store (at level 1) has 975 attack. The best fist I've gotten from the Item World (at level 1) has 62,174 attack.
So, leveling the shop will give you better gear, something you should do to get boots en mass, though the best way to do this is methodically planning and playing yourself.
Use rangers to buff damage dealers that just AoE clear every stage and ideally have enough strength to either kill the boss and AoE or survive an attack. And only finish the stage if the boss drops something.
What makes your method and advice... dangerous is two things.
The first is that you can progress much faster and efficiently by controlling your team. The AI can barely beat stage 20 on a Rank 35 item on auto, whereas I can control it and beat up to stage 60 without getting hit (maybe even higher if I got a legendary to level). It's so effective when done right, I went from struggling to beat a rank 27 to wearing full rank 34 gear in a day moving to the Prism Ranger strategy and controlling it myself.
With that being said, it's also important to note RNG can really screw you when playing like you're playing. Assuming your numbers are factual, it would mean your shop is level 12 (so your knuckles are also rank 12) and your best item is rank 25. This is important, because the item world strategy works well, but it requires constant drops. Here if why that is important. If you don't always progress forward, you drop to your lowest point that can have a drop that you can do. This method might have got you to 25, though you could max out a 25 item, get no drops and then end up back at rank 12 (you can never go lower than your shop rank).
That said, you really should mindlessly grind low level, no innocent items if you want to maximize efficiency, since that levels weapon mastery and skills, but I wouldn't do it for ranks. For those, you should pay attention.